Rule-of-thumb for UV blocking by clouds
They equivalent SPF is less than 5.
What is the UV-blocking from clouds?
A recently published paper helps answer that question. The result is given in terms of the so-called Cloud Modification Factor (CMF), which is the factor by which radiation has been changed due to clouds. If they have no effect, the CMF would be 1. CMFs greater than 1 correspond to situations where radiation scattered from the clouds increases the radiation, while CMF less than 1 correspond to situations where some of the incoming sunlight has been blocked. For example, a CMF of 0.2 means only 20 percent is transmitted, meaning that 80 percent has been blocked.
The figure below shows from that paper sows how the Cloud Modification Factor varies as a function of the Cloud Fraction (CF), as derived from measurements at Davos, Switzerland over a period of two and a half years.
Results are separated according to whether the sun is visible (orange) or blocked by clouds (blue), based on all-sky images and a direct-irradiance threshold. The shaded ribbons show the range of CMFs between the 20th and 80th percentiles. Percentage occurrences for each case (sun-visible or occluded) are shown for three ranges CF ranges (CF 0.0 to 0.3, 0.3 to 0.95, and 0.95 to 1.0). For example, if the cloud fraction is less than 0.3 and not blocking the sun, the CMF is close to 1 for nearly 60 percent of the time. At the other extreme, for cloud cover more than 0.95, when they block the sun the CMF is less than 0.5 for 65 percent of the time.
There are wide variations within each case, but the take-home messages are
If the sun is not obscured (orange), the CMF is close to unity, meaning that the presence of clouds had little effect on the incoming solar radiation.
If the sun is obscured, the CMF is generally close to 0.5, meaning that the clouds typically transmit about half the radiation.
Note that these data are for visible radiation rather than UV wavelengths. However, they are broadly consistent with own results at several sites including Lauder, Boulder, Alice Springs, and Hawaii’s Mauna Loa Observatory, where cloud effects in the UVB region have been shown to be similar to those at longer UVA wavelength. In another study, we showed that cloud effects on sunburning UV are similar to those at visible wavelengths.
It’s worth noting that although clouds do block UV, a significant fraction is still transmitted. Based on these measurements, the average transmission for clouds at Davos would be equivalent to wearing a sunscreen with an SPF of only 2. The lower limit of blue shading of around 0.2 implies that the clouds there would rarely have an equivalent SPF greater than about 5.
Such low SPFs leave plenty of scope for skin damage. Local shade, sunscreens and clothing provide much better protection.
Thanks so much for this Richard! I still see a lot of people saying UVA is relatively constant, implying it's somehow more constant than the wavelengths on either side of it.